Silicon nitride sintered products and processes for their production

ABSTRACT

A silicon nitride sintered product comprising silicon nitride grains and a grain boundary phase, wherein the grain boundary phase consists essentially of a single phase of a Lu 4 Si 2 O 7 N 2  crystal phase, and the composition of the silicon nitride sintered product is a composition in or around a triangle ABC having point A: Si 3 N 4 , point B: 28 mol % SiO 2 -72 mol % Lu 2 O 3  and point C: 16 mol % SiO 2 -84 mol % Lu 2 O 3 , as three apexes, in a ternary system phase diagram of a Si 3 N 4 —SiO 2 —Lu 2 O 3  system. Also disclosed is a silicon nitride sintered product comprising silicon nitride grains and a grain boundary phase of an oxynitride, wherein the composition of the sintered product is a composition in a triangle having point A: Si 3 N 4 , point B: 40 mol % SiO 2 -60 mol % Lu 2 O 3  and point C: 60 mol % SiO 2 -40 mol % Lu 2 O 3 , as three apexes, in a ternary system phase diagram of a Si 3 N 4 —SiO 2 —Lu 2 O 3  system.

This application is a Division of application Ser. No. 09/796,430 Filedon Mar. 2, 2001, now U.S. Pat. No. 6,579,819.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to silicon nitride sintered products andprocesses for their production. More particularly, the present inventionrelates to silicon nitride sintered products which have excellentoxidation resistance and high strength at high temperatures and whichare useful as materials for structural parts of various machines,instruments and equipments including automobiles, mechanical apparatus,chemical apparatus and aerospace equipments, and processes for producingsuch silicon nitride sintered products.

2. Discussion of Background

Sintered products containing silicon nitride as the main component i.e.silicon nitride sintered products are chemically stable at normal orhigh temperatures and have high mechanical strength, and they areexpected to be used for sliding parts such as bearings or engine partssuch as turbocharger rotors.

Heretofore, in order to obtain a silicon nitride sintered product havinghigh strength, an oxide was added as a sintering aid to a siliconnitride powder, followed by firing at a temperature of at least 1,600°C. to carry out liquid phase sintering and densification. Magnesiumoxide, aluminum oxide and oxides of rare earth elements are known asoxides effective as sintering aids. Among them, magnetism oxide,aluminum oxide and yttrium oxide are commonly used alone or in the formof a mixture during the firing. Such a sintering aid reacts with siliconoxide as an oxide layer on the surface of the material at a hightemperature, to form a liquid phase. Sintering proceeds as siliconnitride will diffuse in the liquid phase thus formed. Upon cooling afterthe sintering, the majority of the liquid phase will remain at the grainboundaries in the form of a glass phase, although a part of the liquidphase will be crystallized as an oxide or an oxynitride. Accordingly, asilicon nitride sintered product is usually composed of silicon nitridegrains and a glass phase as the grain boundary phase.

However, when such a sintered product is used in a high temperatureenvironment of at least 1,000° C., there has been a problem that theglass phase at the grain boundaries softens, whereupon the strengthrapidly decreases. The degree of the decrease of strength at a hightemperature depends very much on the chemical composition of the grainboundary phase, as the softening temperature of glass is proportional tothe melting point of the metal-Si—O system in the grain boundary phase.Accordingly, the high temperature strength or creep resistance will behigh when a mixture of aluminum oxide and yttrium oxide is incorporatedrather than when magnesium oxide is incorporated as a sintering aid.

Recently, a study has been made on a system wherein a mixture of a rareearth oxide and silicon oxide, is used as a sintering aid. For example,J. Am. Ceram. Soc. No. 75, p. 2050 (1992) reported on a silicon nitridesintered product having high melting point Y₂Si₂O₇ precipitated at thegrain boundaries by adding a sintering aid of a yttrium oxide-siliconoxide type. In this silicon nitride sintered product, anitrogen-containing apatite (N phase, Y₁₀Si₇O₂₂N₄) or K phase (YSiO₂N)as shown in the phase diagram in FIG. 4, or a glass phase of acomposition close thereto, constitutes a second grain boundary phasefollowing Y₂Si₂O₇. The softening temperature of the N phase or the Kphase is not higher than 1,500° C., whereby the high temperaturestrength or the creep resistance of the silicon nitride sintered producthaving such a phase or a glass phase similar thereto as the grainboundary phase was not fully satisfactory. Further, also in a phasediagram of a Si₃N₄—Y₂Si₂O₂—Si₂N₂O ternary system, compositions in andaround the triangle having the respective components at its apexes werestudied, but the high temperature strengths were not adequate.

Further, with respect to a Si₃N₄—SiO₂-RE₂O₃ (RE: rare earth element)ternary system, JP-A-4-15466 discloses that the J phase (RE₄Si₂O₇N₂),the N phase and the K phase as shown in the phase diagram in FIG. 4,were precipitated at the grain boundaries; JP-A-4-243972 discloses thatthe J phase and a rare earth nitride were precipitated at the grainboundaries; and JP-A-4-292465 discloses that the S phase (RE₂SiO₅) wasprecipitated at the grain boundaries. Further, JP-A-8-48565 disclosesthat the J phase, or two phases i.e. the J phase and the S phase, asshown in the phase diagram in FIG. 5, were precipitated as the grainboundary phase.

However, in each case, it was necessary to add a large amount of a rareearth oxide to control the composition, and the amount of the grainboundary phase increased, whereby there was a new problem that theproduct was susceptible to oxidation at a high temperature, thus leadingto deterioration of the creep resistance and the oxidation resistance.Further, a special heat treatment was required for the crystallization.These publications disclose nothing about the differences in the effectof incorporation among various rare earth elements.

SUMMARY OF THE INVENTION

Under these circumstances, it is an object of the present invention toprovide a silicon nitride sintered product having excellent oxidationresistance and high strength at a high temperature, at the same time,and a process for producing it.

Another object of the present invention is to provide a materialexcellent in creep resistance by efficiently crystallizing a highmelting point sintering assistant at the grain boundaries by means of acommon sintering method requiring no special heat treatment, by studyinga precise composition relating to the type and the amount of the rareearth element, thereby to solve the above-mentioned problems of theprior art.

A sintering aid is required to form a liquid phase at a temperature nothigher than the sintering temperature in order to facilitate the liquidphase sintering and to remain as a crystal phase having a high meltingpoint after the sintering. These two points are essential to obtain asintered product having high heat resistance, and, in many cases, thisis the reason why a rare earth oxide is used as a sintering aid.However, as mentioned above, the phase diagram of a Si₃N₄—SiO₂-RE₂O₃system is complex, and it has been difficult to produce a sinteredproduct having a grain boundary phase composed solely of the desiredhigh melting point phase.

Under the circumstances, the present inventors have paid an attention tothe differences in the phase diagrams and the sinterability amongvarious rare earth elements and have succeeded in letting the J phase(Lu₄Si₂O₇N₂) precipitate at the grain boundaries even by an addition ofa small amount of Lu oxide, by selecting lutetium (Lu) as the rare earthelement and by removing the oxygen impurity in a silicon nitride powderas the starting material, and they have found it possible to obtain asilicon nitride sintered product which has not only high strength butalso excellent oxidation resistance. Likewise, they have succeeded inletting Lu₂SiO₅ phase precipitate efficiently even by a common sinteringmethod requiring no special heat treatment, by selecting lutetium as therare earth element and by controlling the composition precisely, andthey have found it possible to thereby obtain a sintered productexcellent in creep resistance.

Thus, in the first aspect, the present invention provides a siliconnitride sintered product comprising silicon nitride grains and a grainboundary phase, wherein the grain boundary phase consists essentially ofa single phase of a Lu₄Si₂O₇N₂ crystal phase, and the composition of thesilicon nitride sintered product is a composition in or around atriangle ABC having point A: Si₃N₄, point B: 28 mol % SiO₂₋₇₂ mol %Lu₂O₃ and point C: 16 mol % SiO₂₋₈₄ mol % Lu₂O₃, as three apexes, in aternary system phase diagram of a Si₃N₄—SiO₂—Lu₂O₃ system.

In the second aspect, the present invention also provides a siliconnitride sintered product comprising silicon nitride grains and a grainboundary phase of an oxynitride, wherein the composition of the sinteredproduct is a composition in a triangle having point A: Si₃N₄, point B:40 mol % SiO₂-60 mol % Lu₂O₃ and point C: 60 mol % SiO₂-40 mol % Lu₂O₃,as three apexes, in a ternary system phase diagram of a Si₃N₄—SiO₂—Lu₂O₃system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a phase diagram of a Si₃N₄—SiO₂—Lu₂O₃ ternary system showingthe triangle ABC according to the first aspect of the present invention.

FIG. 2 is a phase diagram of a Si₃N₄—SiO₂—Lu₂O₃ ternary system showingthe quadrangle DEFG according to the first aspect of the presentinvention.

FIG. 3 is a schematic view illustrating a transmission electronmicroscopic image showing the microstructure of a silicon nitridesintered product.

FIG. 4 is a phase diagram of a Si₃N₄—SiO₂-RE₂O₃ ternary system such asY₂O₃.

FIG. 5 is a phase diagram of a Si₃N₄—SiO₂-RE₂O₃ ternary system such asLu₂O.

FIG. 6 is a phase diagram of a Si₃N₄—SiO₂—Lu₂O₃ showing the triangle ABCaccording to the second aspect of the present invention.

FIG. 7 is a phase diagram of a Si₃N₄—SiO₂-RE₂O₃ system showing thequadrangle DEFG according to the second aspect of the present invention.

FIG. 8 is a schematic view illustrating the microstructure of thesintered product of Example 10 as observed by a transmission electronmicroscope.

FIG. 9 is a phase diagram of a Si₃N₄—SiO₂-RE₂O₃ system showing thecompositions of the sintered products of Examples 10 to 15 andComparative Examples 4 to 7.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Firstly, the first aspect of the present invention will be described.

For the Si₃N₄—SiO₂-RE₂O₃ ternary system, there are two types of phasediagrams as shown in FIGS. 4 and 5. The type shown in FIG. 4 ischaracterized in that the K phase and the N phase are present, and arare earth element having a large ion radius such as yttrium (Y) belongsto this type. On the other hand, in the phase diagram of the type shownin FIG. 5, these phases are not present, and ytterbium (Yb), thulium(Tm) and lutetium (Lu) belong thereto, as reported in JP-A-8-48565. As aresult of a detailed study by the present inventors on the differencesamong rare elements, Lu was confirmed to belong to the type shown inFIG. 5, and it was found that among rare earth elements, only Lu will becrystallized almost completely at the grain boundaries.

As mentioned above, the chemical composition of the J phase in theSi₃N₄—SiO₂—Lu₂O₃ ternary system is Lu₄Si₂O₇N₂ and has a molar ratio ofSi₃N₄:SiO₂:Lu₂O₃=1:1:4. Accordingly, in order to completely crystallizethe liquid phase formed during the firing to obtain a silicon nitridesintered product, it is necessary to add Lu₂O₃ in an amountcorresponding to four times of SiO₂ in the composition. If the startingmaterial silicon nitride powder contains 1.5 wt % of oxygen as animpurity (an oxygen content of this level or higher is usuallyfrequently observed), such a content corresponds to about 3 wt % (6 mol%) as calculated as SiO₂. Accordingly, in order to make the grainboundary phase a single phase of the J phase, it will be required to addat least 24 mol % of Lu₂O₃. However, if such a large amount of thesintering aid is added, oxidation at a high temperature will befacilitated, thus leading to deterioration of the oxidation resistance,as mentioned above. If the grain boundary phase is not a single phasebut a mixed phase with other phases, the heat resistance tends todeteriorate. In the silicon nitride sintered products reportedheretofore, the grain boundaries are not made of a single phase ofLu₄Si₂O₇N₂, but made of a mixture with other phases in all cases.

Under the circumstances, the present inventors reduced the oxygencontent of the starting material thereby to reduce the amount of Lu₂O₃to be added as a sintering aid and to let a liquid phase having acomposition of Lu₄Si₂O₇N₂ form during the firing, and they succeeded inrealizing a silicon nitride sintered product having a grain boundaryphase made substantially of a single phase of Lu₄Si₂O₇N₂. Here, “madesubstantially of a single phase” means that Lu₄Si₂O₇N₂ is crystallizedcompletely or substantially completely to such an extent that no otherphase is present or other phase is observed only slightly in themicrostructure.

The first aspect of the present invention has been accomplished on thebasis of the above discovery.

Namely, as mentioned above, in the first aspect, the present inventionprovides a silicon nitride sintered product comprising silicon nitridegrains and a grain boundary phase, wherein the grain boundary phaseconsists essentially of a single phase of a Lu₄Si₂O₇N₂ crystal phase,and the composition of the silicon nitride sintered product is acomposition in or around a triangle ABC having point A: Si₃N₄, point B:28 mol % SiO₂₋₇₂ mol % Lu₂O₃ and point C: 16 mol % SiO₂₋₈₄ mol % Lu₂03,as three apexes, in a ternary system phase diagram of a Si₃N₄—SiO₂—Lu₂O₃system.

As preferred embodiments of the first aspect, the present inventionprovides the above silicon nitride sintered product, wherein thecomposition of the silicon nitride sintered product is a composition inor around a quadrangle DEFG having point D: 99 mol % Si₃N₄-0.28 mol %SiO₂-0.72 mol % Lu₂O₃, point E: 99 mol % Si₃N₄-0.16 mol % SiO₂-0.84 mol% Lu₂O₃, point F: 94 mol % Si₃N₄-1.68 mol % SiO₂-4.32 mol % Lu₂O₃ andpoint G: 94 mol % Si₃N₄-0.96 mol % SiO₂-5.04 mol % Lu₂O₃, on thetriangle ABC, as four apexes; the above silicon nitride sinteredproduct, which contains Lu₄Si₂O₇N₂ in an amount of from 2.5 to 10 wt %;the above silicon nitride sintered product, wherein the content ofelements other than Lu, Si, O and N is not more than 1 wt %; and theabove silicon nitride sintered product, wherein at least 90 vol % of thegrain boundary phase present in polycrystal grain boundaries(multi-grain junction) is the Lu₄Si₂O₇N₂ crystal phase.

Further, the present invention provides a process for producing asilicon nitride sintered product, which comprises adding and mixing from1 to 12 wt % of a lutetium oxide powder to a silicon nitride powderhaving an oxygen content of not more than 1.0 wt % and firing themixture at a temperature of from 1,700 to 2,200° C. in a nitrogenatmosphere under a pressure of from 1 to 100 atm. until the compositionas defined above is obtained.

Still further, the present invention provides a process for producing asilicon nitride sintered product, which comprises adding and mixing from1 to 12 wt % of a lutetium oxide powder to a silicon nitride powderhaving an oxygen content of not more than 1.5 wt %, heating the mixtureat a temperature of not higher than 1,600° C. in a nitrogen atmosphereunder a pressure of not higher than 1 atm. to dissipate oxygen until theoxygen content becomes to be the oxygen content of the composition asdefined above, prior to firing, and then firing the mixture at atemperature of from 1,700 to 2,200° C. in a nitrogen atmosphere under apressure of from 1 to 100 atm.

With respect to this process for producing a silicon nitride sinteredproduct, the present invention provides preferred embodiments such thata silicon powder is further added; and the amount of the silicon powderfurther added is from 1 to 10 wt %.

Furthermore, the present invention provides a process for producing asilicon nitride sintered product, which comprises adding and mixing from1 to 12 wt % of a lutetium oxide powder to a silicon powder, then,heating the mixture at a temperature of not higher than 1,500° C. in anitrogen atmosphere to convert silicon to silicon nitride, and then,firing the mixture at a temperature of from 1,700 to 2,200° C. in anitrogen atmosphere under a pressure of from 1 to 100 atm. until thecomposition as defined above is obtained.

With respect to this process for producing a silicon nitride sinteredproduct, the present invention provides a preferred embodiment whereinthe firing is carried out by hot pressing.

As described above, the silicon nitride sintered product of the firstaspect of the present invention is a silicon nitride sintered productcomprising silicon nitride grains and a grain boundary phase, whereinthe grain boundary phase consists essentially of a single phase of aLu₄Si₂O₇N₂ crystal phase, and the composition of the silicon nitridesintered product is a composition in or around a triangle ABC havingpoint A: Si₃N₄, point B: 28 mol % SiO₂-72 mol % Lu₂O₃ and point C: 16mol % SiO₂-84 mol % Lu₂O₃, as three apexes, in a ternary system phasediagram of a Si₃N₄—SiO₂—Lu₂O₃ system as shown in FIG. 1. With acomposition on the SiO₂ side of the side AB, the S phase (Lu₂SiO₅) willprecipitate as a crystal phase, and consequently, the high temperaturestrength will decrease. Further, with a composition on the Lu₂O₃ side ofthe side AC, melilite (Lu₂Si₃O₃N₄) will form as a crystal phase, wherebythe oxidation resistance will decrease.

The composition of the silicon nitride sintered product can be confirmedby a chemical analysis. Specifically, with respect to Si, Lu and othermetal elements, the sintered product is pulverized and then heated anddecomposed in a mixed solution of hydrofluoric acid and nitric acid, andtheir contents in the sintered product are quantitatively analyzed bymeans of a high frequency induction emission spectrophotometer (ICP).oxygen and nitrogen can be quantitatively analyzed by means of atechnique of gas analysis. Namely, the sintered product is heated anddecomposed together with tin and carbon as a combustion improver, andthey can be quantitatively analyzed by the concentrations of nitrogenand carbon monoxide in the decomposed gas. When the composition of thesilicon nitride sintered product according to the first aspect of thepresent invention is in or around a quadrangle DEFG having point D: 99mol % Si₃N₄-0.28 mol % SiO₂-0.72 mol % Lu₂O₃, point E: 99 mol %Si₃N₄-0.16 mol % SiO₂-0.84 mol % Lu₂O₃, point F: 94 mol % Si₃N₄-1.68 mol% SiO₂-4.32 mol % Lu₂O₃ and point G: 94 mol % Si₃N₄-0.96 mol % SiO₂-5.04mol % Lu₂O₃, on the triangle ABC, as four apexes, the oxidationresistance will be further improved. With a composition on the Si₃N₄side of the side DE, the liquid phase component tends to be small,whereby the densification tends to be slightly poor. With a compositionon the Lu₂O₃ side of the side FG, the amount of the sintering aid tendsto be large, whereby the oxidation resistance tends to be poor.

By firing within the above compositional range, the J phase (Lu₄Si₂O₇N₂)will precipitate as a crystal phase at the grain boundaries, yetsubstantially as a single phase.

The content of Lu₄Si₂O₇N₂ in the silicon nitride should properly be from2.5 to 10 wt %. If the content of Lu₄Si₂O₇N₂ is less than 2.5 wt %, thesinterability deteriorates, and the densification tends to hardlyproceed. If it exceeds 10 wt %, the oxidation resistance tends todeteriorate. This J phase can be quantified, for example, from theheight of the peak by the X-ray diffraction method by preparing acalibration curve with powdery mixtures of β-Si₃N₄ and Lu₄Si₂O₇N₂.

Further, the silicon nitride sintered product is particularly excellentin heat resistance when the content of elements other than Lu, Si, O andN is not higher than 1 wt %. If the content of elements other than Lu,Si, O and N exceeds 1 wt %, the heat resistance tends to decrease. Theheat resistance can be improved by the microstructure of the siliconnitride sintered product. Namely, particularly excellent heat resistancecan be realized when a crystal phase of Lu₄Si₂O₇N₂ constitutes at least90 vol % of the grain boundary phase present at the polycrystal grainboundaries as shown in FIG. 3. The rest of the grain boundary phasepresent at the polycrystal grain boundaries, is usually an amorphousphase having a Si—O—N or Lu—Si—O—N composition. If this amorphous phaseexceeds 10 vol %, the heat resistance tends to deteriorate. Thequantitative analysis of the Lu₄Si₂O₇N₂ crystal phase in themicrostructure can be carried out, for example, by cutting out a thinspecimen from the sintered product and observing the polycrystal grainboundaries by means of a transmission electron microscope (TEM).

The process for producing the silicon nitride sintered product accordingto the first aspect of the present invention is characterized mainly byreducing the oxygen content in the starting material, as mentionedabove. However, when a silicon nitride is employed as the startingpowder, the α-form, the β-form, amorphous or a mixture of two or more ofthem, may be employed. There is no particular restriction as to thesize, distribution, shape or purity of the particles. On the other hand,in order to obtain a sintered product having high strength at both roomtemperature and a high temperature, a powder having a particle sizedistribution with a mean particle size of at most 2 μm and a metalimpurity content of not higher than 100 ppm, is preferred. The oxygencontent in the silicon nitride powder to be used as the startingmaterial, is at most 1.5 wt % (provided that the upper limit is 1.0 wt%, in a case where firing is carried out without carrying out theafter-mentioned oxygen dissipation treatment), and the smaller, thebetter. If the oxygen content exceeds the upper limit, the amount of thesintering aid to be added, increases, whereby the oxidation resistancewill be impaired.

As the sintering aid, lutetium oxide Lu₂O₃ is added, and the amount tobe added is an amount to bring about a composition of Lu₄Si₂O₇N₂, whichis specifically from 1 to 12 wt %. Within this range, excellentoxidation resistance can be realized. If the amount of Lu₂O₃ is lessthan 1 wt %, the SiO₂ component tends to be too much in the liquid phaseduring the firing, whereby it tends to be difficult to obtain asubstantial single phase of the Lu₄Si₂O₇N₂ crystal phase. If it exceeds12 wt %, the grain boundary phase will be a single phase of theLu₄Si₂O₇N₂ crystal phase, but the grain boundary phase tends to be toomuch, whereby the oxidation resistance tends to deteriorate. If theamount of Lu₂O₃ is from 2.25 to 9 wt %, the content of Lu₄Si₂O₇N₂ in thesilicon nitride sintered product will be from 2.5 to 10 wt %.

In the process for producing the silicon nitride sintered productaccording to the first aspect of the present invention, the firing iscarried out at a temperature of from 1,700 to 2,200° C. in a nitrogenatmosphere under a pressure of from 1 to 100 atm. The firing method isnot particularly limited. For example, as the simplest method, it ispossible to employ hot pressing. The hot pressing can be carried out,for example, by introducing the starting material powder into a graphitemold, exerting a pressure of from 100 to 500 kg/cm² in a nitrogenatmosphere of from 1 to 100 atm, and firing at a temperature of from1,700 to 2,200° C. for from 30 to 120 minutes.

If the atmosphere is less than 1 atm, silicon nitride tends todecompose, whereby the silicon nitride sintered product will not bedensified. If it exceeds 100 atm, a high pressure gas will be entrappedand will remain as bubbles in the silicon nitride sintered product,whereby it tends to be difficult to densify the product more than 95%.Further, if the firing temperature is lower than 1,700° C., the liquidphase will not be formed sufficiently, whereby densification will notproceed. If it exceeds 2,200° C., the grain growth tends to be vigorous,whereby the room temperature strength tends to deteriorate.

In the process for producing a silicon nitride sintered productaccording to the first aspect of the present invention, in order toreduce the amount of the sintering aid as far as possible thereby tosecure oxidation resistance, oxygen dissipation treatment to furtherreduce the oxygen content in the silicon nitride powder may be carriedout prior to the firing. Specifically, the oxygen dissipation treatmentcan be carried out by heating the starting material powder mixture at atemperature of not higher than 1,600° C. in a nitrogen atmosphere undera pressure of not higher than 1 atm. By this oxygen dissipationtreatment, the oxygen content is reduced to the level of the oxygencontent in the above-mentioned composition of the silicon nitridesintered product. The heating time is usually from 30 to 120 minutes. Bythe oxygen dissipation treatment, the impurity oxygen contained in thesilicon nitride powder will be removed by a reaction such as:

Si₃N₄+3SiO₂=6SiO+2N₂

If the heating temperature exceeds 1,600° C., densification will start,whereby it tends to be difficult to efficiently remove oxygen. Further,if the nitrogen atmosphere exceeds 1 atm, the decomposition reaction asmentioned above tends to hardly proceed, whereby the oxygen removalefficiency deteriorates.

The decomposition reaction as mentioned above will proceed efficientlyif carried out under reduced pressure.

In order to remove oxygen as an impurity from the starting materialsilicon nitride powder, in addition to the above-mentioned oxygendissipation treatment, it is effective to add and mix a silicon powderto the starting material. Added silicon (Si) will react, for example, asfollows, thereby to contribute the removal of oxygen:

Si+SiO₂=2SiO

The addition of the silicon powder varies depending upon the amount ofoxygen contained in the silicon nitride powder, but is usually from 1 to10 wt %. In general, if it is less than 1 wt %, the oxygen removaleffect tends to be poor, and if it exceeds 10 wt %, Si may remain in thesilicon nitride sintered product and is likely to affect the properties.

It may happen that by the oxygen dissipation treatment, densificationtends to hardly proceed. In such a case, densification can be secured,for example, by carrying out the firing by means of hot hydrostaticpressing.

Still further, in the process for producing a silicon nitride sinteredproduct according to the first aspect of the present invention, asilicon powder may be employed as the starting material. As comparedwith a silicon nitride powder, a silicon powder has a small oxygencontent, whereby it will be suitable for forming a liquid phase having aLu₄Si₂O₇N₂ composition while reducing the amount of the sintering aid tobe added. However, when the silicon powder is employed as the startingmaterial, it is necessary to carry out nitriding treatment prior to thefiring. Specifically, after adding and mixing from 1 to 12 wt % of alutetium oxide powder to a silicon powder, the mixture is heated at atemperature of not higher than 1,500° C. in a nitrogen atmosphere. Thisnitriding treatment is a treatment to change silicon to silicon nitrideand is essentially different from the above-mentioned oxygen dissipationtreatment wherein a silicon powder is added in the case where thesilicon nitride powder is used as the starting material. If the heatingtemperature exceeds 1,500° C. in the nitriding treatment, melting ofsilicon will occur, such being undesirable. The nitriding treatment canbe carried out, for example, at least 90% of silicon changes to siliconnitride. Whether or not it has changed to silicon nitride, can beascertained by the X-ray diffraction. After the nitriding treatment, thefiring is carried out at a temperature of from 1,700 to 2,200° C. in anitrogen atmosphere under a pressure of from 1 to 100 atm in the samemanner as in the case where the silicon nitride powder is used as thestarting material. Also in this case, the firing can be carried out byhot pressing.

The silicon nitride sintered product according to the first aspect ofthe present invention will be crystallized in its major portion by usualsintering. However, in order to let crystallization proceed further, itis effective to maintain the product at a temperature of from 1,300 to1,700° C. for from 1 to 24 hours after the sintering.

Now, the second aspect of the present invention will be described.

The RE₂SiO₅ phase which is a silicon oxide of a rare earth element, isan oxide having the highest melting point among oxides of SiO₂-RE₂O₃type, which is preferred as a composition for the grain boundary phase.However, many of rare earth elements will form the H phase (RE₅Si₃O₂N)with a composition in the vicinity thereof. The H phase has a lowmelting point. Accordingly, if a liquid phase composition is designed sothat a RE₂SiO₅ phase will be the grain boundary phase, there has been aproblem that the H phase will partially precipitate, whereby the creepresistance will deteriorate. Further, it has been attempted tocrystallize a mixture of RE₂SiO₅ and RE₂Si₂O₇ at the grain boundaries inorder to avoid such a problem. However, as it contains low melting pointRE₂Si₂O₇, there has been a problem that the heat resistance tends to below.

As mentioned above, there are two types of phase diagrams for aSi₃N₄—SiO₂-Re₂O₃ system, as shown in FIGS. 4 and 5. Here, RE representsa rare earth element, D RE₂Si₂O₇, S RE₂SiO₅, K RESiO₂N, J RE₄Si₂O₇N₂,and M RE₂Si₃O₃N₄. The type shown in FIG. 4 is a type wherein the K phaseand the H phase are present, and RE having a large ion radius such asyttrium, belongs to this type. On the other hand, in the type shown inFIG. 5, such phases are not present, and JP-B-8-48565 discloses that Ybbelongs to this type, Tm and Lu likewise belong to this type.

As mentioned above, as a result of a detailed study on the differencesamong rare earth elements, Lu was found to belong to the type shown inFIG. 5, and it has been found that among crystals of RE₂SiO₅ type of Yb,Tm and Lu, only Lu₂SiO₅ will be completely crystallized. Namely, whensilicon nitride was to be sintered, Lu₂O₃ or a mixture of Lu₂O₃ andSiO₂, was used as a sintering aid, the molar ratio of Lu₂O₃:SiO₂ wasadjusted to about 1:1 in consideration of the oxygen content in thesilicon nitride starting material powder, and the firing was carried outwhile suppressing the amount of oxygen volatilization, whereby asintered product comprising silicon nitride and Lu₂SiO₅ was obtained.

The composition of this sintered product is a composition in a trianglehaving point A: Si₃N₄, point B: 40 mol % SiO₂-60 mol % Lu₂O₃ and pointC: 60 mol % SiO₂-40 mol % Lu₂O₃, as three apexes, in a ternary systemphase diagram of a Si₃N₄—SiO₂—Lu₂O₃ system as shown in FIG. 6. It hasbeen confirmed that at least 90 vol % of the grain boundary phasepresent at the polycrystal grain boundaries, is constituted by theLu₂SiO₅ crystal phase. As the grain boundaries are crystallized almostcompletely, the sintered product is excellent in creep resistance.

Thus, the sintered product according to the second aspect of the presentinvention is a silicon nitride sintered product comprising siliconnitride-grains and a grain boundary phase of an oxynitride, wherein thecomposition of the sintered product is a composition in a trianglehaving point A: Si₃N₄, point B: 40 mol % SiO₂₋₆₀ mol % Lu₂O₃ and pointC: 60 mol % SiO₂-40 mol % Lu₂O₃, as three apexes, in a ternary systemphase diagram of a Si₃N₄—SiO₂—Lu₂O₃ system as shown in FIG. 6.

On the SiO₂ side of the line connecting points A and B, completecrystallization solely of Lu₂SiO₅ can not be accomplished, and othercrystal phases or a glass phase will remain, whereby the creepresistance tends to be low. On the Lu₂O₃ side of the line connectingpoints A and C, other crystal phases or a glass phase will remain,whereby the creep resistance tends to be low.

Further, although the compositional control may tend to be difficult, asa composition whereby higher creep resistance can be obtained, thecomposition of the sintered product is preferably made to be acomposition in a quadrangle having point D: 99 mol % Si₃N₄-0.4 mol %SiO₂-0.6 mol % Lu₂O₃, point E: 99 mol % Si₃N₄-0.6 mol % SiO₂-0.4 mol %Lu₂O₃, point F: 92 mol % Si₃N₄-3.2 mol % SiO₂-4.8 mol % Lu₂O₃ and pointG: 92 mol % SiN₄-4.8 mol % SiO₂-3.2 mol % Lu₂—O₃, as four apexes in aternary system phase diagram of a Si₃N₄—SiO₂—Lu₂O₃ system as shown inFIG. 7.

On the Si₃N₄ side of the line connecting points D and E, the liquidphase component tends to be small, whereby no adequate densification canbe attained. On the SiO₂ side of the line connecting points D and F,complete crystallization can not be accomplished, and a glass phase willremain, whereby the creep resistance tends to be low. On the Lu₂O₃ sideof the line connecting points F and G, the amount of the sintering aidtends to be large, whereby the creep resistance tends to be low. On theLu₂O₃ side of the line connecting points E and G, completecrystallization can not be accomplished, and a glass phase will remain,whereby the creep resistance tends to be low.

In the grain boundary phase, at least 90% is crystallized as Lu₂SiO₅,and the creep resistance can be improved by firing until the proportionof an amorphous phase or a crystal phase other than the Lu₂SiO₅ crystalphase becomes not higher than 10 vol %. If it exceeds 10 vol %, a liquidphase inferior in the heat resistance will form, whereby the creepresistance tends to be low. The firing conditions to satisfy the abovecondition may vary depending upon e.g. the composition. However, thecrystallization rate is higher as the composition is closer to acomposition on the Si₃N₄—Lu₂SiO₅ line in the phase diagram.

The analysis of the composition of the sintered product is carried outby the following method. The obtained sintered product is pulverized andput into a mixed solution of hydrofluoric acid and nitric acid andsubjected to heating and dissolving treatment at 180° C. for 10 hours ina container made of an ethylene trifluoride resin. Then, the Si and Luconcentrations in the solution are quantitatively analyzed by ICP. Then,the sintered product is pulverized and sealed in a tin capsule togetherwith carbon powder and heated and dissolved in a carbon crucible,whereby generated nitrogen and carbon monoxide are quantified toquantitatively analyze the oxygen and nitrogen amounts in the sample.Based on these quantified values, the ratio of Si, Lu, O and N elementsis calculated, and thus, the composition on the phase diagram will beobtained.

The polycrystal grain boundaries of the sintered product of the presentinvention are constituted by crystals of Lu₂SiO₅. With respect tosilicon nitride containing RE₂SiO₅ of a rare earth element, theinfluences of the differences among rare earth elements over the degreeof crystallization and the creep resistance, were investigated indetail, whereby only Lu showed high crystallization at a level of atleast 90% and accomplished high creep resistance. Namely, among variouscompositions of Si₃N₄—SiO₂-RE₂O₃ system, only the composition in thetriangle having point A: Si₃N₄, point B: 40 mol % SiO₂-60 mol % Lu₂O₃and point C: 60 mol % SiO₂-40 mol % Lu₂O₃, accomplished highcrystallization.

Identification of the Lu₂SiO₅ crystal phase is carried out by an X-raydiffraction method, and the formation is confirmed from the heights ofpeaks of β-Si₃N₄, Lu₂SiO₅ and other crystal phases. Further, theproportion of the crystal phase in the polycrystal grain boundaries isdetermined by a transmission electron microscope (TEM). From a sample, athin specimen is cut out and subjected to ion milling treatment, andthen electron diffraction by TEM is carried out, whereby about 50 grainboundaries are observed, and the degree of crystallization is quantifiedby the area ratio of the crystallized portion and the glass phaseportion.

The process for producing the sintered product according to the secondaspect of the present invention is not particularly limited, but thefollowing process may be employed. As the starting material powder, anα-type silicon nitride starting material powder is employed. The amountof an oxygen impurity in the starting material is influential over thecomposition of the grain boundary phase. In the present invention, it isnecessary to precisely control the composition of the final sinteredproduct, and with respect to mixing of the starting material, the oxygenimpurity in the silicon nitride starting material is quantified, and onthe assumption that all of the oxygen impurity is silicon dioxide(SiO₂), the composition is determined as a mixture comprising Si₃N₄ andSiO₂.

As the sintering aid, lutetium oxide Lu₂O₃ or a mixture of lutetiumoxide Lu₂O₃ and silicon dioxide (SiO₂) is added so that the compositionwill be a Lu₂SiO₅ composition. Here, in order to determine thecomposition, it is necessary to determine the starting compositiontaking into consideration the amount of oxygen (SiO₂) contained as animpurity in the silicon nitride starting material and the amount ofoxygen to be dissipated during the firing.

The amount of the sintering aid to be added is determined so that thecomposition of the silicon nitride sintered product will be acomposition in a quadrangle having point D: 99 mol % Si₃N₄-0.4 mol %SiO₂-0.6 mol % Lu₂03, point E:-99 mol % Si₃N₄-0.6 mol % SiO₂-0.4 mol %Lu₂O₃, point F: 92 mol % Si₃N₄-3.2 mol % SiO₂-4.8 mol % Lu₂O₃ and pointG: 92 mol % SiN₄-4.8 mol % SiO₂-3.2 mol % Lu₂O₃, as four apexes in aternary system phase diagram of a Si₃N₄—SiO₂—Lu₂O₃ system.

The sintering method is not particularly limited, but it is preferred tocarry out firing at a temperature of from 1,700 to 2,000° C. in nitrogenunder a pressure of from 2 to 100 atm. If the pressure is less than 2atm, silicon nitride and silicon dioxide are likely to react anddecompose during the firing, whereby the compositional variation tendsto be large, and a dense sintered product can hardly be obtained. If thepressure exceeds 100 atm, a high pressure gas is likely to be entrappedin the sintered product, whereby voids will be formed in the sinteredproduct, whereby a dense sintered product can hardly be obtained.

Most simply, the firing is carried out by hot pressing under a nitrogengas pressure. For example, it is a method wherein the starting powder isput into a graphite mold, and a pressure of from 100 to 500 kg/cm² isexerted in nitrogen under a pressure of from 2 to 10 atm, and firing iscarried out at a temperature of from 1,700 to 2,000° C. for from 30 to120 minutes. If the atmosphere is less than 2 atm, volatilization ofoxygen in the sintered product tends to be vigorous, whereby thecompositional control tends to be difficult. If the atmosphere exceeds10 atm, a high pressure gas is likely to be entrapped and will remain inthe silicon nitride sintered product, whereby densification will notproceed beyond 95%. If the firing temperature is lower than 1,700° C.,no adequate liquid phase will form, whereby no adequate densificationcan be accomplished. If the firing temperature exceeds 2,000° C., thegrain growth tends to be vigorous, whereby the room temperature strengthtends to be low.

The amount of oxygen volatilization during the firing is preferably suchthat the amount of reduction calculated as SiO₂ will be not more than40% of the total of SiO₂ contained in the silicon nitride startingmaterial and SiO₂ added. If the amount of oxygen volatilization exceedsthis level, the compositional change during the firing tends to belarge, whereby crystallization of Lu₂SiO₅ beyond 90% will be difficult.

Now, the present invention will be described in further detail withreference to Examples and Comparative Examples. However, it should beunderstood that the present invention is by no means restricted by suchspecific Examples.

Firstly, the silicon nitride sintered product and the process for itsproduction according to the first aspect of the present invention willbe described with reference to Examples and Comparative Examples.

TABLE 1 Composition of Composition of starting material Composition ofsintered sintered product powder mixture Firing condition product asquantified on phase diagram Example Si₃N₄ Lu₂O₃ Temp. Time Lu Si O NSi₃N₄ SiO₂ Lu₂O₃ No. Powder wt % wt % Pattern ° C. hr wt % wt % wt % wt% mol % mol % mol % 1 P1 91.82 8.18 S1 1800 1 7.34 55.01 1.16 36.4996.20 0.70 3.10 2 P3 98.72 1.28 S2 1900 2 1.15 59.27 0.19 39.39 98.400.14 0.46 3 P3 92.65 7.35 S2 1800 2 6.58 55.52 1.09 36.82 96.40 0.842.76 4 P3 93.35 6.65 S2 1800 2 5.98 55.94 0.97 37.11 96.80 0.70 2.50 5P1 94.84 5.16 S2 2000 1 4.64 56.87 0.74 37.75 97.60 0.48 1.92 6 P1 93.856.15 S2 2000 1 5.52 56.22 1.01 37.24 96.00 1.12 2.28 7 P1 88.75 11.25 S22000 1 10.11 53.11 1.57 35.21 94.80 0.84 4.36 8 P1 93.73 2.44 S2 2000 12.08 58.63 0.33 38.96 99.00 0.19 0.81 9 5.58 S1 1800 1 3.02 57.99 0.4638.54 98.60 0.19 1.21

TABLE 2 Room temp. 1500° C. oxidation characteristics of 1500° C. testsintered product bending Weight Example Porosity Strength strengthincrease Strength No. % MPa MPa mg/cm² MPa 1 1.2 1050 820 0.05 1020 21.5 1130 840 0.1 940 3 0.8 1010 780 0.08 950 4 0.5 1350 770 0.12 1010 51.3 1050 760 0.09 880 6 1.4 1030 870 0.08 850 7 0.7 1220 790 0.06 1050 81.6 980 820 0.02 750 9 1.2 970 870 0.01 840

TABLE 3 Composition of Composition of Compar- starting materialComposition of sintered sintered product ative powder mixture Firingcondition product as quantified on phase diagram Example Si₃N₄ Lu₂O₃Pattern Temp. Time Lu Si O N Si₃N₄ SiO₂ Lu₂O₃ No. Powder wt % wt % S1 °C. hr wt % wt % wt % wt % mol % mol % mol % 1 P2 1800 1 2 P2 92.56 7.44S2 1700 1 6.69 55.43 1.18 36.71 96.00 1.20 2.80 3 P1 99.67 0.33 S1 21002 0.3 59.8 0.24 39.65 99.00 0.88 0.12

TABLE 4 Room temp. 1500° C. oxidation characteristics of 1500° C. testFormed Proportion of Comparative sintered product bending Weight phasesby J phase by Example Porosity Strength strength increase Strength X-rayTEM No. % MPa MPa mg/cm² MPa diffraction % 1 1.5 1220 470 0.15 950 J(Yb)100 2 12 480 350 8.3 310 J.M 70 3 25 350 300 50.5 220 J.M 75

EXAMPLE 1

Lutetium oxide was added in an amount of 8.18 wt % to a silicon nitridepowder (powder P1) having a mean particle size of 0.5 μm, an oxygencontent of 1.0 wt % and an a-type content of 92%, followed by mixing andpulverization for 2 hours by means of a wet system ball mill havingethanol added. Then, the mixture was dried in air by a rotary evaporatorand then formed into a molded product of 80 mm×45 mm×10 mm by moldingunder 20 MPa.

This molded product was put into a graphite mold and fired by means of agas pressure hot pressing furnace. Firstly, it was heated in a vacuum of10⁻² Pa from room temperature to 1,300° C. at a rate of 500° C. per hourand then held at 1,300° C. for 1 hour, whereupon nitrogen gas of 10 atmwas introduced into the furnace to exert a pressure of 20 MPa, and thetemperature was raised to 1,800° C. at a rate of 500° C. per hour andmaintained at 1,800° C. for 1 hour. This firing condition is identifiedas pattern S1 in Table 1.

The obtained sintered product was pulverized and put into a mixedsolution of hydrofluoric acid and nitric acid, and heating anddissolving treatment was carried out by maintaining the solution at 180°C. for 10 hours in a Teflon container, whereupon the Si and Luconcentrations in the solution were quantified by means of a highfrequency induction emission spectrophotometer (ICP). Then, thepulverized product of the sintered product was sealed in a tin capsuletogether with carbon powder and heat-melted in a carbon crucible,whereby generated nitrogen and carbon monoxide were quantified toquantitatively analyze the oxygen amount and the nitrogen amount in thesintered product.

As shown in Table 1, the quantified values were Lu:7.3375 wt %,Si:55.011 wt %, O:1.1586 wt %, and N:36.493 wt %. From this result, thecomposition of the sintered product was 96.20 mol % Si₃N₄0.703 mol %SiO₂-3.097 mol % Lu₂O₃, and it was a composition in the triangle ABC andin the quadrangle DEFG shown in FIGS. 1 and 2, respectively.

Further, from the results of the X-ray diffraction, the phases formed inthe sintered product were found to be β-Si₃N₄ and Lu₄Si₂O₇N₂.

Further, from the sintered product, a thin specimen was cut out andsubjected to argon ion milling treatment, whereupon it was observed bymeans of a transmission electron microscope (TEM). As shown in FIG. 3, amicrostructure comprising β-type silicon nitride grains, and two-crystalgrain boundaries and polycrystal grain boundaries, were observed. At thepolycrystal grain boundaries, crystallization of Lu₄Si₂O₇N₂ wasconfirmed by the electron diffraction. In all of randomly selected 20polycrystal grain boundaries, Lu₄Si₂O₇N₂ was crystallized. Then, thesintered product was subjected to surface grinding by means of a 800mesh diamond wheel and processed into a size of 3 mm×4 mm×40 mm,whereupon the bending strength was measured by room temperature and hightemperature four point bending in accordance with JIS R1601. As shown inTable 2, the porosity of the sintered product was 1.2%, the roomtemperature four point bending strength was 1,050 MPa, and the hightemperature four point bending strength at 1,500° C. was 820 MPa.Further, the test specimen after processing was heated to 1,500° C. in afurnace of atmospheric air and maintained for 100 hours. After thisoxidation test, the weight increase was 0.05 mg/cm², and the roomtemperature four point bending strength was 1,020 MPa. The oxidationresistance is evaluated by the weight change and the room temperaturestrength when heated in air. If oxidation proceeds, an oxide film willform, whereby a weight increase and a decrease in strength will beobserved.

From the foregoing results, the obtained sintered product is judged tobe a silicon nitride sintered product which has adequately high strengtheven at a high temperature and which at the same time has excellentoxidation resistance.

COMPARATIVE EXAMPLE 1

Ytterbium oxide was added in an amount of 12 wt % to a silicon nitridepowder (power P2) having a mean particle size of 0.3 μm, an oxygencontent of 1.8 wt % and an α-type content of 90%, followed by mixing andpulverizing in the same manner as in Example 1, and the mixture wasformed into a molded product.

This molded product was put into a graphite mold and fired by means of agas pressure hot pressing furnace. Firstly, it was heated in a vacuum of10⁻² Pa from room temperature to 800° C. at a rate of 500° C. per hour,and then nitrogen gas of 10 atm was introduced into the furnace to exerta pressure of 20 MPa, and the temperature was raised to 1,800° C. at arate of 500° C. per hour and maintained at 1,800° C. for 1 hour. Fromthe results of the X-ray diffraction, the formed phases were confirmedto be β-Si₃N₄ and Yb₄Si₂O₇N₂. Further, from the observation by means ofTEM, the grain boundary phase was confirmed to be entirely Yb₄Si₂O₇N₂.

With respect to the obtained sintered product, the bending strength wasmeasured in the same manner as in Example 1. As a result, as shown inTable 4, the porosity was 1.5%, the room temperature four point bendingstrength was 1,220 MPa, and the high temperature four point bendingstrength at 1,500° C. was 470 MPa. As compared with Example 1,deterioration in the high temperature strength was observed. Thedeterioration is considered to be attributable to the fact that thegrain boundary phase is not Lu₄Si₂O₇N₂ but is Yb₄Si₂O₇N₂, from thecomparison with Example 1.

EXAMPLES 2 to 7

As shown in Table 1, as the silicon nitride powder, powder P1 which wasthe same silicon nitride powder as used in Example 1 or a siliconnitride powder (powder P3) having a mean particle size of 0.8 μm, anoxygen content of 0.8 wt % and an α-type content of 95%, was used, andruthenium oxide was added and mixed thereto in the same manner as inExample 1 in the amount as identified in Table 1, and the mixture wasformed into a molded product.

The molded product was put into a graphite mold and fired by means of agas pressure hot pressing furnace. As the firing condition, pattern S2different from Example 1 was employed. Namely, the molded product washeated in a vacuum of 10⁻² Pa from room temperature to 1,300° C. at arate of 500° C. per hour, then nitrogen gas of 0.8 atm was introducedinto the furnace at 1,300° C. Then, it was held at 1,500° C. for 1 hour,and then nitrogen gas of 10 atm was introduced into the furnace to exerta pressure of 20 MPa, and the temperature was raised to a level shown inTable 1 at a rate of 500° C. per hour and maintained at thattemperature.

The obtained sintered product was subjected to the quantitative analysesof Si and Lu and quantitative analyses of the oxygen amount and thenitrogen amount, in the same manner as in Example 1.

The quantified values and the compositions of the sintered productsbased thereon are as shown in Table 1, and each composition was acomposition in the triangle ABC and the quadrangle DEFG shown in FIGS. 1and 2; respectively.

Further, from the results of the X-ray diffraction, the formed phaseswere confirmed to be all β-Si₃N₄ and Lu₄Si₂O₇N₂. Further, as a result ofthe TEM observation, a microstructure comprising β-type silicon nitridegrains, and two-crystal grain boundaries and polycrystal grainboundaries, was observed, and from the electron diffraction,crystallization of Lu₄Si₂O₇N₂ was confirmed at the polycrystal grainboundaries. At all of randomly selected 20 polycrystal grain boundaries,Lu₄Si₂O₇N₂ was crystallized.

And, the bending test and the oxidation test were carried out in thesame manner as in Example 1. The porosity, the room temperature fourpoint bending strength and the high temperature four point bendingstrength at 1,500° C., were as shown in Table 2.

Also in Examples 2 to 7, the obtained sintered products are judged to besilicon nitride sintered products which have adequately high strengtheven at a high temperature and which at the same time have excellentoxidation resistance.

COMPARATIVE EXAMPLES 2 and 3

As shown in Table 3, as the silicon nitride powder, powder P1 or powderP2 employed in Example 1 or Comparative Example 1, respectively, wasemployed, and lutetium oxide was added and mixed thereto in the samemanner as in Example 1 in an amount as shown in Table 3, and the mixturewas formed into a molded product.

This molded product was put into a graphite mold and fired by means of agas pressure hot pressing furnace. As the firing condition, pattern S1or pattern S2 employed in Example 1 or Examples 2 to 7, respectively,was employed.

The obtained sintered product was subjected to quantitative analyses ofSi and Lu and quantitative analyses of the oxygen amount and thenitrogen amount, in the same manner as in Example 1.

The quantified values and the compositions of the sintered productsbased thereon were as shown in Table 3. The compositions were dislocatedtowards the Lu₂O₃ side of the composition around the triangle ABC andthe quadrangle DEFG shown in FIGS. 1 and 2, respectively. Thecomposition of the grain boundary phase became a Lu₂O₃ excessivecomposition, and from the result of the X-ray diffraction, it wasconfirmed that the M phase was formed other than Lu₄Si₂O₇N₂.

The bending test and the oxidation test were carried out in the samemanner as in Example 1. The porosity, the room temperature four pointbending strength and the high temperature four point bending strength at1,500° C., were as shown in Table 4.

With the composition wherein the M phase will form, the viscosity of theliquid phase is high, whereby the sinterability decreases, and noadequate densification can be accomplished. Accordingly, the porositywas high, and consequently, both the room temperature strength and thehigh temperature strength became low. Further, due to the high porosity,the oxidation resistance also decreased.

EXAMPLE 8

As shown in Table 1, as the silicon nitride powder, powder P1 which wasthe same as the silicon nitride powder as used in Example 1, wasemployed, and 4.84 wt % of a silicon powder and 2.435 wt % of lutetiumoxide were added thereto and mixed in the same manner as in Example 1,and the mixture was formed into a molded product.

This molded product was put into a graphite mold and fired by pattern S2using a gas pressure hot pressing furnace under the firing conditions asused in Examples 2 to 7.

The obtained sintered product was subjected to quantitative analyses ofSi and Lu and quantitative analyses of the oxygen amount and thenitrogen amount, in the same manner as in Example 1. As shown in Table1, the quantified values were Lu:2.0774 wt %, Si:58.63 wt %, O:0.3286 wt%, and N:38.964 wt %. From this result, the composition of the sinteredproduct was found to be 99.00 mol % Si₃N₄-0.19 mol % SiO₂-0.81 mol %Lu₂O₃, and is a composition in the triangle ABC and the quadrangle DEFGshown in FIGS. 1 and 2, respectively.

Further, from the results of the X-ray diffraction, the phases formed inthe sintered product were confirmed to be β-Si₃N₄ and Lu₄Si₂O₇N₂.

Further from the results of the TEM observation, a microstructurecomprising β-type silicon nitride grains, and two crystal grainboundaries and polycrystal grain boundaries, was observed, and from theelectron diffraction, crystallization of Lu₄Si₂O₇N₂ was confirmed at thepolycrystal grain boundaries. At all of randomly selected 20 polycrystalgrain boundaries, Lu₄Si₂O₇N₂ was crystallized.

And, the bending test and the oxidation test were carried out in thesame manner as in Example 1. As shown in Table 2, the porosity of thesintered product was 1.6%, the room temperature four point bendingstrength was 980 MPa, and the high temperature four point bendingstrength at 1,500° C. was 820 MPa. After the oxidation test, the weightincrease was 0.02 mg/cm², and the room temperature four point bendingstrength was 750 MPa.

Also in Example 8, the obtained sintered product is judged to be asilicon nitride sintered product which has adequately high strength evenat a high temperature and which at the same time has excellent oxidationresistance.

EXAMPLE 9

Lutetium oxide was added in an amount of 5.58 wt % to a silicon powderhaving an average particle size of 0.8 μm, followed by mixing andpulverizing in the same manner as in Example 1, and then, the mixturewas formed into a molded product.

This molded product was heated from room temperature to 1,200° C. in avacuum of 10⁻² Pa, and then it was heated to 1,400° C. at a rate of 10°C. per hour and maintained at 1,400° C. for 24 hours to carry outnitriding treatment. Thereafter, the molded product was put into agraphite mold and fired in the same manner as in Example 1 under thefiring condition of pattern S1.

The obtained sintered product was subjected to quantitative analyses ofSi and Lu and quantitative analyses of the oxygen amount and thenitrogen amount, in the same manner as in Example 1. As shown in Table1, the quantified values were Lu:3.02 wt %, Si:57.99 wt %, 0:0.457 wt %,and N:38.538 wt %. From this result, the composition of the sinteredproduct was found to be 98.60 mol % Si₃N_(4-0.19) mol % SiO_(2-1.21) mol% Lu₂O₃ and is a composition in the triangle ABC and the quadrangle DEFGshown in FIGS. 1 and 2, respectively.

Further, from the results of the X-ray diffraction, the phases formed inthe sintered product were confirmed to be β-Si₃N₄ and Lu₄Si₂O₇N₂.

Further, as a result of the TEM observation, a microstructure comprisingβ-type silicon nitride grains, and two crystal grain boundaries andpolycrystal grain boundaries, was observed, and from the electrondiffraction, crystallization of Lu₄Si₂O₇N₂ was confirmed at thepolycrystal grain boundaries. At all of randomly selected 20 polycrystalgrain boundaries, Lu₄Si₂O₇N₂ was crystallized.

And, the bending strength and the oxidation test were carried out in thesame manner as in Example 1. As shown in Table 2, the porosity of thesintered product was 1.2%, the room temperature four point bendingstrength was 970 MPa, and the high temperature four point bendingstrength at 1,500° C. was 870 MPa. After the oxidation test, the weightincrease was 0.01 mg/cm², and the room temperature four point bendingstrength was 840 MPa.

Also in Example 9, the obtained sintered product is judged to be asilicon nitride sintered product which has adequately high strength evenat a high temperature and which at the same time has excellent oxidationresistance.

The first aspect of the present invention is by no means limited by theforgoing practical Embodiments and Examples. Various modifications arepossible with respect to details such as the size, the distribution, theshape, the purity, etc. of the starting material powder, the firingconditions, etc.

As described in detail in the foregoing, according to the first aspectof the present invention, a silicon nitride sintered product which hashigh strength at a high temperature and which at the same time hasexcellent oxidation resistance, can be provided.

Now, the second aspect of the present invention will be described indetail with reference to Examples and Comparative Examples.

EXAMPLE 10

Lutetium oxide was added in an amount of 6.9 wt % to a silicon nitridepowder (powder P4) having a mean particle size of 0.51 μm, an oxygencontent of 0.93 wt % and an a-type content of 92%, followed by mixingand pulverizing for 2 hours by means of a wet system ball mill havingethanol added. The starting material powder contained 0.93 wt % of anoxygen impurity, and accordingly, the composition of the startingmaterial powder was calculated as 98.2 wt % Si₃N₄-1.8 wt % SiO₂, wherebythe real composition of the mixture was 91.52 wt % Si₃N₄-1.78 wt %SiO₂-6.7 wt % Lu₂O₃, as shown in Table 5(B). Then, the powder was driedin air by means of a rotary evaporator and formed into a molded productof 80 mm×45 mm×10 mm by molding under a pressure of 200 kg/cm².

TABLE 5(A) Composition of starting material mixture (wt %) Example No.Si₃N₄ SiO₂ Lu₂O₃ 10 93.2 0.1 6.7 11 90.2 0.5 9.3 12 95.5 4.5 13 91.5 0.77.8 14 90.5 0.2 9.3 15 88.6 0.1 11.3 Comparative 96 0.6 3.4 Example 4Comparative 89 1.8 9.2 Example 5 Comparative 85.3 14.7 Example 6Comparative 82.5 0.3 17.2 Example 7

TABLE 5(B) Real composition of starting material (wt %) Example No.Si₃N₄ SiO₂ Lu₂O₃ 10 91.52 1.78 6.7 11 88.58 2.12 9.3 12 93.78 1.72 4.513 89.85 2.35 7.8 14 88.87 1.83 9.3 15 87.01 1.69 11.3 Comparative 94.272.33 3.4 Example 4 Comparative 87.4 3.4 9.2 Example 5 Comparative 83.761.54 14.7 Example 6 Comparative 81.01 1.79 17.2 Example 7

This molded product was put into a graphite mold and fired by means of agas pressure hot pressing furnace.

Firstly, it was heated in a vacuum of 10⁻² Pa from temperature to 1,300°C. at a rate of 500° C. per hour, and nitrogen gas of 10 atm wasintroduced at this temperature to exert a pressure of 200 kg/cm², andthe temperature was raised to 1,800° C. at a rate of 500° C. per hourand maintained at 1,800° C. for 1 hour.

The obtained sintered product was pulverized, then put into a mixedsolution of hydrofluoric acid and nitric acid, and subjected to heatingand dissolving treatment at 180° C. for 10 hours in a container made ofa ethylene tetrafluoride resin, whereupon the Si and Lu concentrationsin the solution were quantified by ICP. Then, the sintered product waspulverized, sealed in a tin capsule together with carbon powder andheated and dissolved in a carbon crucible, whereby generated nitrogenand carbon monoxide were quantified to quantitatively analyze the oxygenamount and the nitrogen amount in the sample.

The quantified values were Lu:6.05 wt %, Si:55.79 wt %, 0:1.38 wt % andN: 36.78 wt %. The oxygen amount decreased during the firing, and thereduction calculated as SiO₂ was 38%. From these results, thecomposition of the sintered product was found to be 95 mol % Si₃N₄-2.5mol % SiO₂-2.5 mol % Lu₂O₃ and was a composition in ABC and DEFG inFIGS. 6 and 7, respectively. Further, as a result of the X-raydiffraction, the formed phases were β-Si₃N₄ and Lu₄Si₂O₇N₂.

The obtained sintered product was subjected to surface grinding by a 800mesh diamond wheel and processed into a shape of 3 mm×4 mm×10 mm,followed by pressing under a compression pressure of 300 MPa to obtainthe creep deformation rate. The deformation rate was 1.2×10⁻⁸ persecond, thus showing excellent creep resistance.

From the sintered product, a thin specimen was cut out and subjected toargon ion milling treatment, whereupon it was observed by a transmissionelectron microscope, whereby as shown in FIG. 3, it had a microstructurecomprising silicon nitride grains, and two crystal grain boundaries andpolycrystal grain boundaries. Further, with respect to the polycrystalgrain boundaries (point A), electron diffraction was carried out todetermine the crystal phase of the grain boundary phase.

Further, the polycrystal grain boundaries were investigated by theelectron diffraction, whereby crystallization of Lu₂SiO₅ was observed.Further, randomly selected 50 polycrystal grain boundaries wereinvestigated, whereby at all of the 50 grain boundaries, Lu₂SiO₅ wascrystallized. Thus, by the formation of the grain boundary phase havingthe Lu₂SiO₅ composition, a silicon nitride sintered product having creepresistance at a high temperature, was obtained.

EXAMPLES 11 to 15

Lutetium oxide and silicon dioxide were added to the powder P4 in theamounts as identified in Table 5(A), and a molded product was preparedin the same manner as in Example 10. The real composition of thestarting material is shown in Table 5 (B). This molded product was putinto a graphite mold and fired by means of a gas pressure hot pressingfurnace. Firstly, it was heated in a vacuum of 10⁻² Pa from roomtemperature to 1,300° C. at a rate of 500° C. per hour, and thennitrogen gas under a pressure as identified in Table 6(A) was introducedat this temperature to exert a pressure of 200 kg/cm², and thetemperature was raised to a level as identified in Table 6(A) at a rateof 500° C. per hour and maintained for a period of time as identified inTable 6(A).

TABLE 6(A) Firing condition Temp. Time Gas pressure Example No. ° C. hratm 10 1800 1 10 11 1850 1 10 12 2000 1 10 13 1800 2 2 14 1950 1 5 151800 2 4 Comparative 1800 1 1 Example 4 Comparative 1800 1 10 Example 5Comparative 1800 1 10 Example 6 Comparative 1800 1 20 Example 7

TABLE 6(B) Change in the amount of SiO₂ during the firing Before Afterfiring firing Dissipated Dissipation Example No. (wt %) (wt %) amount(wt %) rate (%) 10 1.78 1.08 0.7 39 11 2.12 1.48 0.64 30 12 1.72 1.010.71 41 13 2.35 1.75 0.6 26 14 1.83 1.06 0.77 42 15 1.69 1.13 0.56 33Comparative 2.33 1.63 0.7 30 Example 4 Comparative 3.4 2.8 0.6 18Example 5 Comparative 1.54 0.94 0.6 39 Example 6 Comparative 1.79 1.320.47 26 Example 7

With respect to the obtained sintered product, the oxygen and nitrogenamounts in the sample were quantified in the same manner as in Example10. The quantified values were as shown in Table 7(A). The oxygen amountdecreased during the firing, and the reduction rate calculated as SiO₂was as shown in Table 6 (B). From these results, the composition of thesintered product was accurately calculated, and the calculated valuesare as shown in Table 7(B), and the respective compositions werecompositions in ABC and DEFG in the phase diagram in FIG. 9.

TABLE 7(A) Composition of sintered product (mol %) Example No. Si₃N₄SiO₂ Lu₂O₃ 10 95 2.6 2.4 11 93 3.6 3.4 12 96 2.4 1.6 13 93 4.2 2.8 14 942.6 3.4 15 93 2.8 4.2 Comparative 95 3.8 1.2 Example 4 Comparative 906.7 3.3 Example 5 Comparative 92 2.4 5.6 Example 6 Comparative 90 3.46.6 Example 7

TABLE 7(B) Real composition of sintered product (wt %) Example No. Si LuO N 10 55.94 5.82 1.37 36.87 11 54.3 8.14 1.9 35.65 12 57.24 3.93 1.0837.75 13 55.18 6.8 1.86 36.15 14 54.39 8.1 1.68 35.84 15 53.17 9.87 1.9635 Comparative 57.8 2.99 1.28 37.93 Example 4 Comparative 54.19 8.05 2.635.16 Example 5 Comparative 51.17 12.83 2.26 33.74 Example 6 Comparative49.68 14.94 2.75 32.63 Example 7

Further, as a result of the X-ray diffraction, the formed phases wereβ-Si₃N₄ and Lu₂SiO₅. The obtained sintered product was subjected tosurface grinding by a 800 mesh diamond wheel and processed into shape of3 mm×4 mm×10 mm, followed by pressing under a compression pressure of300 MPa to obtain the creep deformation rate. The deformation rate wasas shown in Table 8, which indicates excellent creep resistance.

From the sintered product, a thin specimen was cut out and subjected toargon ion milling treatment, whereupon it was observed by a transmissionelectron microscope, whereby it had a microstructure comprising siliconnitride grains, and two crystal grain boundaries and polycrystal grainboundaries. Further, the polycrystal grain boundaries were investigatedby the electron diffraction, whereby it was found that Lu₂SiO₅ wascrystallized. Further, randomly selected 50 polycrystal grain boundarieswere investigated, whereby the proportion of the Lu₂SiO₅ crystal was asshown in Table 8.

Thus, by forming a grain boundary phase having the Lu₂SiO₅ composition,a silicon nitride sintered product hardly susceptible to creepdeformation at a high temperature, was obtained. Thus, by forming thegrain boundary phase having a Lu₂SiO₅ composition, a silicon nitridesintered product excellent in creep resistance without no substantialdeterioration of the strength at a high temperature, was obtained.

COMPARATIVE EXAMPLES 4 to 7

Lutetium oxide and silicon dioxide were added to the powder P4 in theamounts as identified in Table 5(A), followed by mixing andpulverization for 2 hours by means of a wet system ball mill havingethanol added. The starting material powder contained 0.93 wt % of anoxygen impurity, and the real composition of the mixture was as shown inTable 5(B). Then, the mixture was dried in air by means of a rotaryevaporator, and then a molded product of 80 mm×45 mm×10 mm was obtainedby molding under a pressure of 20 MPa.

This molded product was put into a graphite mold and fired by means of agas pressure hot pressing furnace. Firstly, it was heated in a vacuum of10⁻² Pa from room temperature to 1,300° C. at a rate of 500° C. perhour, and then nitrogen gas under a pressure as identified in Table 6(A)was introduced at this temperature to exert a pressure of 200 kg/cm²,and the temperature was raised to a level as identified in Table 6(A) ata rate of 500° C. per hour and maintained for a period of timeidentified in Table 6(A).

With respect to the obtained sintered product, the compositionalanalysis was carried out in the same manner as in Example 10, wherebythe quantified values were as shown in Table 7(A). The oxygen amountdecreased during the firing, and the reduction rate calculated as SiO₂was as shown in Table 7(B). From these results, the composition of thesintered product was accurately calculated, and the real composition wasas shown in Table 7(B), and it was a composition outside of DEFG in thephase diagram of FIG. 9. Further, as a result of the X-ray diffraction,the formed phases were as shown in Table 8.

TABLE 8 Characteristics of sintered product Number and proportion ofeach phase in 50 grain boundary phases Proportion Creep rate Type ofExample No. Per second of RE RE₂SiO₅ RE₂SiO₇ RE₄Si₂O₇N₂ AmorphousRE₂SiO₅ (%) 10 1.2 × 0.00000001 Lu 50 100 11 2.5 × 0.00000001 Lu 48 2 9612 0.5 × 0.00000001 Lu 48 2 96 13   2 × 0.00000001 Lu 46 4 92 14 3.5 ×0.00000001 Lu 47 3 94 15   4 × 0.00000001 Lu 45 5 90 Comparative  15 ×0.00000001 Lu 8 30 12 16 Example 4 Comparative  24 × 0.00000001 Lu 2 408 4 Example 5 Comparative  28 × 0.00000001 Lu 5 31 14 10 Example 6Comparative  20 × 0.00000001 Lu 6 29 15 12 Example 7 Comparative  36 ×0.00000001 Y 31 8 11 62 Example 8 Comparative  43 × 0.00000001 Nd 25 1213 50 Example 9 Comparative  52 × 0.00000001 Ce 12 38 24 Example 10Comparative  18 × 0.00000001 Yb 40 10 80 Example 11

The creep deformation rate obtained under a compression pressure of 300MPa in the same manner as in Example 10, was as shown in Table 8, andthe creep resistance was poor. It was observed by a transmissionelectron microscope in the same manner as in Example 10, whereby theproportion of the Lu₂SiO₅ crystals was as shown in Table 8.

Comparative Example 4 is a composition on the SiO₂ side than the line EGin the phase diagram in FIG. 9, and the degree of crystallization ofLu₂SiO₅ is low, whereby creep resistance is poor. Comparative Example 5is a composition on the SiO₂ side of the line EG, and the amount of thesintering aid is also large. Accordingly, not only the degree ofcrystallization of Lu₂SiO₅ is low, but also the amount of amorphoussubstance increases, whereby creep resistance is poor. ComparativeExample 6 is a composition on the Lu₂O₃ side of the line DF, and thedegree of crystallization of Lu₂SiO₅ is low, whereby creep resistance ispoor. Comparative Example 7 is a composition on the Lu₂O₃ side of theline DF, and the amount of the sintering aid is large. Accordingly, notonly the degree of crystallization of Lu₂SiO₅ is low but also the amountof amorphous substance is large, whereby creep resistance is poor.

Thus, with compositions outside DEFG of the phase diagram in FIG. 9, thedegree of crystallization of Lu₂SiO₅ was low, and the degree of creepdeformation at a high temperature was large.

COMPARATIVE EXAMPLES 8 to 11

A rare earth oxide (RE: rare earth element) and silicon dioxide wereadded to the powder P4 to form a composition (Si₃N₄-5 mol % RE-2SiO₅) asidentified in Table 9, and a molded product was obtained in the samemanner as in Example 10.

TABLE 9 Mixed composition of starting material powders (wt %) Si₃N₄ SiO₂Y₂O3 La₂O₃ CeO₂ Yb₂O₃ Comparative 96 4 Example 8  9 94.4 5.6 10 94.1 5.911 93.3 6.7

This molded product was put into a graphite mold and fired by means of agas pressure hot pressing furnace. Firstly, it was heated in a vacuum of10⁻² Pa from room temperature to 1,300° C. at a rate of 500° C. perhour, and nitrogen gas under a pressure of 10 atm was introduced at thistemperature to exert a pressure of 200 kg/cm², and the temperature wasraised to 1,800° C. at a rate of 500° C. per hour and maintained at1,800° C. for 1 hour.

The formed phases of the obtained sintered product were as shown inTable 8. The creep deformation rate obtained under a compressionpressure of 300 MPa in the same manner as in Example 10 was as shown inTable 8, and the creep resistance was poor. The sintered product wasobserved by a transmission electron microscope in the same manner as inExample 10, whereby the proportion of RE₂SiO₅ crystal was as shown inTable 8.

Thus, even with compositions in DEFG of the phase diagram in FIG. 7, ifrare earth elements other than Lu were employed, the degree ofcrystallization of RE₂SiO₅ was low, and the degree of creep deformationat a high temperature was large.

A silicon nitride sintered product was a material poor in the heatresistance, since a sintering aid made of an oxide remained atpolycrystal grain boundaries after sintering, whereby creep resistancewas low. As described in the foregoing, according to the second aspectof the present invention, a sintered product comprising silicon nitrideand a grain boundary phase having a Lu₂SiO₅ composition is prepared bycontrolling the firing conditions and the amount and the type of thesintering aid to be added to the silicon nitride starting materialpowder, whereby it is possible to obtain a material in which the grainboundaries are completely crystallized, and thus it is possible toprovide a material excellent in the creep resistance.

What is claimed is:
 1. A silicon nitride sintered product comprisingsilicon nitride grains and a grain boundary phase, wherein thecomposition of the sintered product is a composition within a trianglehaving point A: 100 mol % Si₃N₄, point B: 40 mol % SiO₂-60 mol % Lu₂O₃and point C: 60 mol % SiO₂-40 mol % Lu₂O₃, as three apexes, in a ternarysystem phase diagram of a Si₃N₄—SiO₂—Lu₂O₃ system.
 2. A silicon nitridesintered product comprising silicon nitride grains and a grain boundaryphase, wherein the composition of the silicon nitride sintered productis a composition in a quadrangle having point D: 99 mol % Si₃N₄-0.4 mol% SiO₂-0.6 mol % Lu₂O₃, point E: 99 mol % Si₃N₄-0.6 mol % SiO₂-0.4 mol %Lu₂O₃, point F: 92 mol % Si₃N₄-3.2 mol % SiO₂-4.8 mol % Lu₂O₃ and pointG: 92 mol % SiN₄-4.8 mol % SiO₂-3.2 mol % Lu₂O₃, as four apexes in aternary system phase diagram of a Si₃N₄—SiO₂—Lu₂O₃ system.
 3. Thesilicon nitride sintered product according to claim 1, wherein the grainboundary phase is a Lu₂SiO₅ crystal phase.
 4. The silicon nitridesintered product according to claim 3, wherein the proportion of anamorphous phase or a crystal phase other than the Lu₂SiO₅ crystal phasein the grain boundary phase is not more than 10 vol % of the volume ofthe grain boundary phase.
 5. A process for producing a silicon nitridesintered product, the process comprising adding and mixing a lutetiumoxide powder, or a mixture of a lutetium oxide powder and silicondioxide, to a silicon nitride starting material powder, so that thecomposition after firing is a composition within a triangle having pointA: 100 mol % Si₃N₄, point B: 40 mol % SiO₂-60 mol % Lu₂O₃ and point C:60 mol % SiO₂-40 mol % Lu₂O₃, as three apexes, in a ternary system phasediagram of a Si₃N₄—SiO₂—Lu₂O₃ system, firing the mixture at atemperature of from 1,700 to 2,000° C. in a nitrogen atmosphere under apressure of from 2 to 100 atm, and producing the silicon nitridesintered product of claim
 1. 6. The process according to claim 5,wherein a volatile amount of SiO₂ during the firing is not higher than40% of the total SiO₂, as the weight reduction calculated as SiO₂. 7.The process according to claim 5, wherein the firing is carried out byhot pressing in a nitrogen gas atmosphere under a pressure in a range offrom 2 to 10 atm.